The overall objective is to better understand the cellular and subcellular aspects of immune responses in fish. Most of the proposed work is focused on studying the steps involved in the induction and regulation of in vitro antibody responses to TD and TI antigens with the channel catfish, a teleost fish of considerable economic importance and previously shown to have B cells, T cells and accessory cells akin to those in higher animals. Experiments are proposed to answer five important basic questions yet to be approached in any fish species. 1) Do fish have separable helper and suppressor cells which interact to regulate immune responses? 2) Is antigen processing and/or presentation necessary for fish immune responses, and if so, what cell(s) performs this function? 3) Do fish immune reponses involve the participation of a spectrum of interleukins, such as Il-1, IL-2, and BCGF(s)? 4) What are the critical early events involved in the activation of fish lymphocytes for mitogenesis and subsequent differentiation? 5) Do fish T and B cell plasma membranes exhibit different structural and functional properties? The answers to these questions will be evaluated for their reactive contributions, if any, to low temperature iommunosuppression of fish primary antibody responses to TD antigens. Completion of these proposed studies will not only provide heretofore lacking information on the comparative or phylogenetic aspects of specific immune responses but may also explain the well established but poorly understood phenomenon of low temperature immunosuppression seen in all ectothermic (cold blooded) vertebrates. Furthermore the ability to manipulate fish immune responses by physiologically relevant environmental temperature changes adds a novel dimension to studying interactions within the immune system of fish that is not feasible with the more commonly studied higher endothermic animals.